ROSAT Guest Observer Facility

ROSAT Status Report #113:

Dec 12, 1994



Comments on the WGACAT/ROSATSRC Comparison Report by MPE

N.E. White, L. Angelini and P. Giommi


In the report from MPE on a comparison of the wgacat and rosatsrc catalogs (see below) they report several significant differences. Probably the most critical is an offset in the positions between wgacat and simbad. Investigation by us confirms there is a problem in the WGACAT positions, with a 6 arc sec offset in the inner region and 15 arc sec in the outer. These offsets are comparable to the quoted uncertainties in the ROSAT boresight, which is one reason we had not noticed it before. If you have been using the default 1 arc min search radius in browse, it is not likely to be a problem. We have traced the source of the error and a corrected catalog is being generated. We hope to have it released within the next 2-3 weeks. This will also contain additional sources from new fields that have recently entered the archive. Here is a rule of thumb to correct the positions. Its not a simple scaling and depends on whether the source is less than 7860,7860 in detector coords. If the X pixel coord is less than 7860 then to the RA ADD 4 arc sec in the inner region, and 10 arc sec in the outer (denoted by the source designation being _in or _out). If the y coord is less than 7860 SUBTRACT from the dec 4 arc sec and 10 arc sec if it is, respectively, in the inner or outer regions. In addition to this, there is a overall offset of order 5 arc sec in the outer region and 2 arc sec in the inner. These latter corrections should be applied in that sense. The other points mentioned are an excess of faint sources between 16 and 20 arc sec in wgacat. There is an overlap in the separate runs made for the inner and outer regions, close to the inner rib. In the overlap region the source detect for the outer region is more sensitive due to an overall lower background. We would recommend that all _out sources within OFFSET < 19 arc sec be treated with caution. The differences in the vignetting are much as expected, since we used a simple approximation. On the other side, the ROSATSRC count rates in the outer region are wrong because they did not include the psf correction.


MPE Report


Dear ROSAT observer,

following the report of a comparison of the two ROSAT PSPC source catalogs WGACAT and ROSATSRC. The report and compressed postscript files of the figures can be copied from the anonymous ftp account rosat_svc.mpe-garching.mpg.de in the directory archive/sourcecat file wga_rosatsrc.doc.


Differences in the two ROSAT catalogs of pointed PSPC observations

Haberl F., Pietsch W., and Voges W.
Max-Planck-Institut fuer extraterrestrische Physik


We have compared the two catalogs of ROSAT sources (ROSATSRC, Voges W., Gruber R., Haberl F., Kuerster M., Pietsch W., and Zimmermann U., ROSAT NEWS No. 32; and WGACAT, White N.E., Giommi P., and Angelini L., IAU Circular 6100) and find differences with respect to

Source positions,
Source distribution at different off-axis angles,
Detector vignetting, and
Count rates.

This study was not done in depth yet, and should not be considered as being complete, but the first results are so important in our view, that we want to bring it to the attention of the ROSAT community at this early stage. We are continuing this work and will update this information when available.

A short summary of the results is given in the following:

Source positions

The WGACAT and ROSATSRC catalogs have been correlated with SIMBAD stars which have a position accuracy of better than 1.5 arcsec. For the ROSATSRC catalog 68% of the matching sources are found within 17 arcsec to the SIMBAD position (Fig. 1)

In Fig. 1 one can even resolve the two peaks in declination separated by about 20 arcsec. This is caused by the boresight correction which was wrong by about 10 arcsec in the first processed revision 0 data. It also leads to a slight broadening of the distribution in right ascension. The improved boresight values used in revision 1 moves the peaks closer together (residual boresight error less than about 5 arcsec) and the source catalog can be used to further improve the boresight correction.

Fig. 1a and 2a show the same correlation for sources in the inner part of the detector (within 19 arcmin off-axis angle) and Fig. 1b and 2b for the outer part.

The distribution of distances to SIMBAD sources found for the WGACAT shows a systematic shift of about 5 arcsec in RA and about 15 arcsec in DEC. Moreover the distribution is asymmetric (Fig. 2). From an inspection of several images with plotted source positions (we used the FITS data from the archive and standard EXSAS procedures for creating images and overlaying the source positions) from both catalogs the shift between the source positions is visible by eye and is always in the same direction in the image (Fig. 3 and 4). The shift is larger in the outer part of the image where the images for the WGACAT have larger image binning. In the description of the WGACAT similar images and overlays of the same ROSAT pointing are presented. There are no systematic offsets between sources in the image and the overlay. This suggests an intrinsic problem connected to the relation between ROSAT image pixel and sky position in the XIMAGE package.

Source distribution at different off-axis angles

The number of sources found at different off-axis angles is very different in the WGACAT and ROSATSRC catalogs. While in the inner part of the detector the maximum-likelihood method used in the ROSATSRC is more sensitive and finds more sources than the sliding window method in the WGACAT, the number of sources in the WGACAT shows a strong peak between 16 and 20 arcmin (Fig. 5 and 6). For the WGACAT the images were split into two parts; the inner part inside 19 arcmin and the outer part outside 18 arcmin off-axis angle. The expected double identification due to the 1 arcmin overlap can not be responsible for the whole peak and the nature of the additional sources is not clear.

The peak and a higher number of sources outside 22 arcmin, present in the WGACAT, are very prominent for source count rates below 0.04 cts/s (Fig. 7 and 8) while absent for count rates above 0.04 cts/s (Fig. 9 and 10). Also the distribution of sources which are common to both catalogs does not show the increased number of weak and spurious sources around the support ring (Fig. 11), indicating that a considerably higher number of weak sources is contained in the WGACAT between about 16 and 40 arcmin off-axis angle. (The absolute numbers in Fig. 11 are higher as they contain also multiple matches.)

Detector vignetting

The vignetting correction for the ROSATSRC sources is calculated per photon and then averaged. In the inner part of the detector where ML was running, this is done also energy dependent. Fig. 12 shows the vignetting correction as function of off-axis angle. For the WGACAT the vignetting correction is determined from the position of the source only and therefore shows no scatter. The vignetting correction in the WGACAT is energy independent and lies below the values derived from the official effective area calibration file. The difference increases with off-axis angle and is about 2% at 10 arcmin up to about 10% at 50 arcmin, and is even larger for hard sources.

Count rates

In the outer part of the detector the count rates in the ROSATSRC catalog are not corrected for the point spread function and give only the counts inside the cut radius, corrected for vignetting. Therefore the count rates can be significantly too low and cases were found with a factor of two discrepancy. Further investigations in this direction are in progress.

Postscript files of figures:
Fig. 1: mpe_simbad.ps.gz
Fig. 2: wga_simbad.ps.gz
Fig. 1a: mpe_simbad_in.ps.gz
Fig. 2a: wga_simbad_in.ps.gz
Fig. 1b: mpe_simbad_out.ps.gz
Fig. 2b: wga_simbad_out.ps.gz
Fig. 3: us200008_full.ps.gz
Fig. 4: us200008_in.ps.gz The Pleiades Center pointing also shown in the WGA catalog description; the crosses denote the ROSATSRC positions, the circles the WGACAT positions from the inner part images and the squares the WGACAT positions from the outer part images
Fig. 5: mpe_number_offaxis.ps.gz
Fig. 6: wga_number_offaxis.ps.gz
Fig. 7: mpe_weak_number_offaxis.ps.gz
Fig. 8: wga_weak_number_offaxis.ps.gz
Fig. 9: mpe_strong_number_offaxis.ps.gz
Fig.10: wga_strong_number_offaxis.ps.gz
Fig.11: wga_mpe_number_offaxis.ps.gz
Fig.12: mpe_vign_offaxis.ps.gz previous Status Report #112

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